1. Field of the Invention
The disclosure generally relates to power management methods and systems for medium access control over wireless networks, such as ad-hoc wireless networks, where each power-saving station can dynamically tune its duty cycle and timing synchronization among power-saving stations is not necessary.
2. Description of the Related Art
IEEE 802.11 is currently the most popular international MAC (Medium Access Control) standard for WLANs (Wireless Local Area Networks). Based on the network architecture, the WLANs can be classified into “infrastructure WLANs” and “ad hoc wireless networks.” FIG. 1 is a schematic diagram illustrating an ad hoc network, where each station (110, 120, 130, 140 and 150) can communicate with adjacent stations via radio.
FIG. 2 is a schematic diagram illustrating a power consumption model for a general wireless network interface card installed in a station. Each station can be in exact one of transmitting, receiving, listening, or doze states. From FIG. 2, we can see that if a station wants to save energy, it should enter the doze state. In the doze state, a station cannot send and receive, but consumes very low power.
In IEEE 802.11 power management for ad hoc networks, time is divided into fixed-sized beacon intervals (BIs for short). Mobile station operating in the power-saving (PS for short) mode should wake up prior to each TBTT (target beacon transmission time) and wait for a random backoff time to contend for broadcasting a beacon frame, which is mainly used for timing synchronization. All power-saving (PS for short) station should remain awake during the entire ATIM (Announcement Traffic Indication Message) window. If a station does not send or receive ATIM frames during the ATIM window, then that station may enter the doze state after the end of the ATIM window. If a station receives a directed ATIM frame during the ATIM window, then that station should reply the ATIM ACK (Acknowledgement) and remains awake during the entire beacon interval (BI for short). After the end of the ATIM window, the source station uses the DCF (Distributed Coordination Function) procedure to send buffered data frames to intended destination, and that destination should acknowledge it receipt. For a more detailed presentation, please refer to the IEEE 802.11 specification.
FIG. 3 is a schematic diagram illustrating an example of power management in an ad hoc network based on IEEE 802.11. As shown in FIG. 3, stations X and Y compete to broadcast a beacon frame for timing synchronization when BI 1 begins. Once station Y successfully broadcasts the beacon frame, other stations (including Y) cancel their beacon transmissions. On the other hand, since both stations X and Y do not send and receive any ATIM frames during the ATIM window (AW for short), they enter the doze state after the AW ends. During the AW of BI 2, station Y sends the ATIM frame (denoted “A” in FIG. 3) to station X and X replies an ATIM ACK (denoted “a” in FIG. 3). After the AW ends, both stations X and Y remain awake. In addition, station Y transmits a data frame (denoted “D” in FIG. 3) to station X, and X immediately replies a data ACK (denoted “d” in FIG. 3) to Y.
However, the mechanism for timing synchronization in IEEE 802.11 is not perfect. For example, simulation results show that if the number of stations in an ad hoc wireless network is 200, the average clock offset among the stations is about 220 us, and the maximum clock offset may approach about 500 us. If the number of stations in an ad hoc wireless network is 500, the average clock offset among the stations is about 264 us, and the maximum clock offset may approach about 600 us. Once the clock offset among stations in the network is large enough, the IEEE 802.11 power management mechanism may completely fails. For example, in FIG. 4, we can see that, when the clock offset (ΔT) between stations X and Y lies in between AW and BI-AW, PS stations X and Y forever lose each other's beacon and ATIM frames. (Note that “PS” stands for “power-saving”.) This also implies that stations X and Y can never send data frames to each other when they are operating in the PS mode.
Conventional asynchronous power management protocols require that all PS stations must have the same SRI (schedule repetition interval), where the SRI is defined as “the consecutive beacon intervals that comprise some different awake/sleep schedules repeat at regular intervals”. This implies that all PS stations will have the same idle duty cycle, where idle duty cycle is defined as the fraction of time during which the radio is on and there is no data traffic. Obviously, such a requirement (that all PS stations must have the same SRI) is impractical since we hope that each PS station can dynamically tune its idle duty cycle according to its current residual battery power or other quality-of-service (QoS for short) considerations. Worse, such a requirement requires that all WLAN NIC vendors adopt the same value of SRI. We know that a PS station with a larger SRI value can save more energy, but may suffer a longer data reception delay. Thus it is very difficult for NIC vendors to determine the best SRI value for all different kinds of environments. Worst, all known asynchronous power management protocols may completely fail when some PS stations have different values of SRI. More specifically, two PS neighbors in asynchronous environment may forever lose each other's beacon and ATIM frames when they have different SRI values.